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10632 Chem. Commun., 2013, 49, 10632--10634 This journal is c The Royal Society of Chemistry 2013 Cite this: Chem. Commun., 2013, 49, 10632 Enantioselective synthesis of a-nitro-d-ketosulfones via a quinine–squaramide catalyzed conjugate addition of a-nitrosulfones to enones† Kalisankar Bera and Irishi N. N. Namboothiri* Conjugate addition of a-nitrosulfones to vinyl ketones in the presence of 0.2 mol% of a quinine–squaramide organocatalyst afforded a-nitro- d-ketosulfones possessing a tetrasubstituted chiral center in excellent yield and enantioselectivity in most cases. This strategy also offers a facile and convenient entry into c-sulfonylhydroxamates that are one carbon homologs of potent enzyme inhibitors. Sulfonylhydroxamic acids are inhibitors of matrix metalloprotease (MMP) and are effective for the treatment of cancer and arthritis. 1 For instance, b-sulfonylhydroxamate 1a, in which a sulfonyl group is attached to a chiral center, is a potent MMP and PDE (phospho- diesterase) inhibitor (Fig. 1). 2,3 The enantiopure compound 1a displays superior activity as compared to the racemic one and the enantioselective synthesis of 1a involves the oxidation of an enantioenriched thioether precursor. 3 Recently, sulfones attached to a tetrasubstituted chiral center 4–6 received considerable attention due to their wide range of biological activities, for instance, against Alzheimer’s (as g-secretase inhibitor 1b) 4 and glaucoma. 6 The sulfonyl group in organosulfones, viz. active methylene sulfones, vinyl sulfones and other sulfone based nucleophiles and electrophiles, takes part in a variety of synthetic transformations and is an easily removable functional group. 7 Enantioselective approaches to sulfones include catalytic asymmetric Michael addition of various nucleophiles to b-unsubsti- tuted vinyl sulfones 8 and b-substituted vinyl sulfones, 9 nucleophilic addition of active methylene 10 and methine 11,12 sulfones to various electrophiles, and other miscellaneous methods. 13,14 However, to our knowledge, generation of sulfones attached to a chiral carbon remains scarcely explored under catalytic asymmetric conditions. 12,14 As part of our ongoing research program to develop novel catalytic methods for the asymmetric synthesis of quaternary carbon centers, we have reported enantioselective synthesis of quaternary a-nitro/aminophosphonates. 15,16 Herein we report an organocatalyzed enantioselective synthesis of quaternary a-nitro- d-ketosulfones via Michael addition of a-nitrosulfones to enones using an alkaloid derived squaramide catalyst. Transformations of the product to quaternary g-nitro-g-sulfonyl hydroxamic acid and g-nitro-g-sulfonyl carboxylic acid are also reported here. The reaction conditions were optimized by performing Michael addition of nitrosulfone 3a to phenyl vinyl ketone 2a using several cinchona-based bifunctional organocatalysts and solvents at different temperatures (Fig. 2 and Table 1). At the outset, the quinine–thiourea catalyst C1, recently reported from our labora- tory, 15 was screened (entry 1). To our delight, the Michael adduct 4a, a quaternary a-nitrosulfone, was isolated in good yield (96%) and enantioselectivity (90% ee) when 10 mol% of C1 was employed in mesitylene at rt (entry 1). Other catalysts such as quinine–thiourea C2, cinchonidine–thiourea C3 and cinchonine–thiourea C4 were also quite effective in providing the Michael adduct 4a in excellent yield and selectivity (entries 2–4). Encouraged by these results, further improvement in the enantioselectivity was explored by employing bifunctional cinchona–squaramide catalysts C5–C7 with greater H-bonding capability (entries 5–8). 17 Cinchonidine–squaramide catalyst C5 provided product 4a with lower selectivity (entry 5). However, quinine–squaramide catalyst C6 18 furnished the Michael adduct 4a with improved selectivity (94%) and excellent yield Fig. 1 Potent inhibitors of MMP, PDE and g-secretase. Fig. 2 Catalysts screened. Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400 076, India. E-mail: [email protected] † Electronic supplementary information (ESI) available. CCDC 946574. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc45985c Received 5th August 2013, Accepted 25th September 2013 DOI: 10.1039/c3cc45985c www.rsc.org/chemcomm ChemComm COMMUNICATION Open Access Article. Published on 26 September 2013. Downloaded on 2/17/2022 9:30:38 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue
Transcript of RSC CC C3CC45985C 3.
10632 Chem. Commun., 2013, 49, 10632--10634 This journal is c The Royal Society of Chemistry 2013
Cite this: Chem. Commun.,2013,49, 10632
Enantioselective synthesis of a-nitro-d-ketosulfones viaa quinine–squaramide catalyzed conjugate addition ofa-nitrosulfones to enones†
Kalisankar Bera and Irishi N. N. Namboothiri*
Conjugate addition of a-nitrosulfones to vinyl ketones in the presence
of 0.2 mol% of a quinine–squaramide organocatalyst afforded a-nitro-
d-ketosulfones possessing a tetrasubstituted chiral center in excellent
yield and enantioselectivity in most cases. This strategy also offers a
facile and convenient entry into c-sulfonylhydroxamates that are one
carbon homologs of potent enzyme inhibitors.
Sulfonylhydroxamic acids are inhibitors of matrix metalloprotease(MMP) and are effective for the treatment of cancer and arthritis.1
For instance, b-sulfonylhydroxamate 1a, in which a sulfonyl groupis attached to a chiral center, is a potent MMP and PDE (phospho-diesterase) inhibitor (Fig. 1).2,3 The enantiopure compound 1adisplays superior activity as compared to the racemic one and theenantioselective synthesis of 1a involves the oxidation of anenantioenriched thioether precursor.3 Recently, sulfones attachedto a tetrasubstituted chiral center4–6 received considerable attentiondue to their wide range of biological activities, for instance, againstAlzheimer’s (as g-secretase inhibitor 1b)4 and glaucoma.6
The sulfonyl group in organosulfones, viz. active methylenesulfones, vinyl sulfones and other sulfone based nucleophiles andelectrophiles, takes part in a variety of synthetic transformationsand is an easily removable functional group.7
Enantioselective approaches to sulfones include catalyticasymmetric Michael addition of various nucleophiles to b-unsubsti-tuted vinyl sulfones8 and b-substituted vinyl sulfones,9 nucleophilicaddition of active methylene10 and methine11,12 sulfones to variouselectrophiles, and other miscellaneous methods.13,14 However, to our
knowledge, generation of sulfones attached to a chiral carbonremains scarcely explored under catalytic asymmetric conditions.12,14
As part of our ongoing research program to develop novelcatalytic methods for the asymmetric synthesis of quaternarycarbon centers, we have reported enantioselective synthesis ofquaternary a-nitro/aminophosphonates.15,16 Herein we report anorganocatalyzed enantioselective synthesis of quaternary a-nitro-d-ketosulfones via Michael addition of a-nitrosulfones to enonesusing an alkaloid derived squaramide catalyst. Transformationsof the product to quaternary g-nitro-g-sulfonyl hydroxamic acidand g-nitro-g-sulfonyl carboxylic acid are also reported here.
The reaction conditions were optimized by performing Michaeladdition of nitrosulfone 3a to phenyl vinyl ketone 2a using severalcinchona-based bifunctional organocatalysts and solvents atdifferent temperatures (Fig. 2 and Table 1). At the outset, thequinine–thiourea catalyst C1, recently reported from our labora-tory,15 was screened (entry 1). To our delight, the Michael adduct4a, a quaternary a-nitrosulfone, was isolated in good yield (96%) andenantioselectivity (90% ee) when 10 mol% of C1 was employed inmesitylene at rt (entry 1). Other catalysts such as quinine–thioureaC2, cinchonidine–thiourea C3 and cinchonine–thiourea C4 were alsoquite effective in providing the Michael adduct 4a in excellent yieldand selectivity (entries 2–4). Encouraged by these results, furtherimprovement in the enantioselectivity was explored by employingbifunctional cinchona–squaramide catalysts C5–C7 with greaterH-bonding capability (entries 5–8).17 Cinchonidine–squaramidecatalyst C5 provided product 4a with lower selectivity (entry 5).However, quinine–squaramide catalyst C618 furnished the Michaeladduct 4a with improved selectivity (94%) and excellent yield
Fig. 1 Potent inhibitors of MMP, PDE and g-secretase.
Fig. 2 Catalysts screened.
Department of Chemistry, Indian Institute of Technology Bombay,
Mumbai 400 076, India. E-mail: [email protected]
† Electronic supplementary information (ESI) available. CCDC 946574. For ESI andcrystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc45985c
Received 5th August 2013,Accepted 25th September 2013
DOI: 10.1039/c3cc45985c
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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 10632--10634 10633
(98%, entry 6). At this juncture, possible enhancement of enantio-selectivity by screening various solvents at different temperatures inthe presence of 10 mol% of C6 was investigated (entries 8–17). Thus,toluene was identified as the best solvent which at �60 1C providedMichael adduct 4a in 98% yield and 99% ee (entry 17). We wereamazed to note that high yield (98%) and selectivity (99% ee) wereunaffected even when the catalyst loading was gradually reducedto 5, 2 and 0.2 mol% albeit at the expense of the reaction rate(entries 18–20).
The scope of the above reaction was subsequently investigatedby treating a-nitrosulfone 3a with various enones 2b–o under theabove optimized conditions, i.e. 0.2 mol% catalyst C6, in toluene at�60 1C (Table 2). It is noteworthy that regardless of the steric andelectronic properties and the position of substituents on thearomatic ring of enones, the Michael adducts 4a–i were isolatedin excellent yields (97–99%) and selectivities (96–99% ee) over aperiod of 2–20 h (entries 1–9). However, a faster reaction rate wasobserved in the case of enones possessing electron withdrawingsubstituents such as NO2, CN, Br and Cl at unhindered positions(entries 5–7 and 9) when compared to enones possessing electrondonating substituents such as Me and OMe (entries 2–4). Polycyclicaromatic enones 2j–k, heteroaromatic enones 2l–m and an alipha-tic enone 2n were also well tolerated in terms of the chemical andoptical yields of the products (entries 10–14), except that 1-naphthylvinyl ketone 2k furnished the Michael adduct 4k with lowerselectivity (72% ee, entry 11). However, since the reaction ratedramatically decreased when using aliphatic enone 2n the reactionhad to be conducted at rt with 10 mol% of the catalyst C6 (entry 14).A dienone 2o also furnished the Michael adduct 4o in 98%yield and 97% ee (entry 15). Notably, the reaction is highlyregioselective in that a-nitrosulfone 3a selectively reacted with the
b-unsubstituted olefin moiety in the presence of a b-substitutedolefin moiety in enone 2o (entry 15).
Encouraged by the excellent results obtained from the reac-tion of nitrosulfone 3a with a variety of enones 2 (Table 2), thescope of the reaction was investigated further with other steri-cally and electronically different nitrosulfones 3b–g (Table 3).Thus, various alkyl, allyl and benzyl substituted nitrosulfones3b–g were treated with a representative enone 2i under theoptimal reaction conditions (Table 3). In general, the Michaeladducts 5a–f were obtained in excellent yields (90–99%) andenantioselectivities (entries 1–6). The enantioselectivity droppedmarginally (to 85% ee) in the case of 5a (entry 1) and substan-tially (to 50% ee) when a nitrosulfone 3f possessing a long alkylchain was employed (entry 5). This is attributable to the inter-ference of the long, linear and flexible alkyl chain in 3f in theMichael addition step (see Fig. 3, vide infra). Since the rate of thereaction was very slow at�60 1C in the case of benzyl substitutednitrosulfone 3g, the reaction was performed at rt (entry 6).
The absolute configuration of the Michael adducts 4–5 wasunambiguously assigned as R by single crystal X-ray structure
Table 1 Catalyst screening and optimization of reaction conditionsa
Entry Cat. Solvent T (1C) Time (h) Yieldb (%) eec (%)
1 C1 Mesitylene rt 1 96 902 C2 Mesitylene rt 1 94 883 C3 Mesitylene rt 1 95 914 C4 Mesitylene rt 1 93 87d
5 C5 Mesitylene rt 1 95 806 C6 Mesitylene rt 1 98 947 C7 Mesitylene rt 1 98 948 C6 Xylene rt 1 97 939 C6 Toluene rt 1 97 9410 C6 PhCF3 rt 1 95 8811 C6 CH2Cl2 rt 1 86 8612 C6 (CH2)2Cl2 rt 1 97 8513 C6 THF rt 1 91 9214 C6 Diethyl ether rt 1 92 8615 C6 MeCN rt 1 92 7016 C6 Toluene �20 2 98 96.417 C6 Toluene �60 4 98 9918e C6 Toluene �60 5 98 98.519f C6 Toluene �60 8 98 98.620g C6 Toluene �60 11 98 99
a The reactions were performed at the 0.2 mmol scale. b After silica gelcolumn chromatography. c ee’s determined by chiral HPLC. d Oppositeenantiomer. e 5 mol% catalyst. f 2 mol% catalyst. g 0.2 mol% catalystand 0.5 mmol of 3a.
Table 2 Scope of enones 2a
Entry R Time (h) 4 Yieldb (%) eec (%)
1 C6H5 11 4a 98 992 4-MeC6H4 10 4b 99 963 4-OMeC6H4 16 4c 98 994 3,4-(OMe)2C6H3 20 4d 99 965 4-ClC6H4 5 4e 98 >996 4-CNC6H4 3 4f 99 >997 4-NO2C6H4 2 4g 99 998 2-ClC6H4 14 4h 97 989 3-BrC6H4 5 4i 99 >99d
10 2-Naphthyl 14 4j 98 9911 1-Naphthyl 15 4k 99 7212 2-Furyl 12 4l 96 9913 2-Thienyl 7 4m 97 9714e c-C6H11 10 4n 86 9115 PhCHQCH 15 4o 98 97
a The reactions were performed at the 0.5 mmol scale. b After silica gelcolumn chromatography. c ee determined by chiral HPLC. d This reactionwas also performed on a larger scale (vide infra). e Reaction performed at rt.
Table 3 Scope of nitrosulfones 3a
Entry R1 3 Time (h) 5 Yieldb (%) eec (%)
1 Et 3b 12 5a 96 852 Allyl 3c 10 5b 99 >993 n-Bu 3d 14 5c 93 954 n-C5H11 3e 17 5d 98 955 n-C7H15 3f 17 5e 90 506d Benzyl 3g 8 5f 95 96
a The reactions were performed at the 0.5 mmol scale. b After silica gelcolumn chromatography. c ee determined by chiral HPLC. d Reactionperformed at rt.
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10634 Chem. Commun., 2013, 49, 10632--10634 This journal is c The Royal Society of Chemistry 2013
analysis of a representative compound 4i (Fig. 3, see also the ESI†).The proposed mechanism based on model studies involves depro-tonation of nitrosulfone 3 by the quinuclidine moiety of catalyst C6and activation of enone 2 by the squaramide moiety (Fig. 3). In thefavored approach II, the squaramide moiety also co-ordinates withthe nitro group, and the quinuclidine moiety with the nitro and thesulfonyl groups, to enable Re-face approach of the enone 2 towardsthe nitrosulfone anion affording (R)-nitrosulfone 4 or 5. Thealternative approach I appears to be disfavored due to severe stericinteractions between the phenyl group of the sulfone and thequinuclidine moiety of the catalyst.
Our reaction conditions are suitable for the synthesis of nitro-sulfones 4 and 5 on a multi-gram scale without any appreciabledrop in the yield or selectivity. This was demonstrated by thesynthesis of 2.9 g of 4i (97%) with 99% ee via Michael addition of1.5 g of nitrosulfone 3a to 2.2 g of vinyl ketone 2i (Table 2, entry 9).
Nitrosulfonylketones 4 and 5 in which the carbonyl group is atthe d-position of the nitro and the sulfonyl group are excellentprecursors for the enantioselective synthesis of carboxylic acid 7 andhydroxamic acid 8 (Scheme 1). Thus a representative nitrosulfonyl-ketone 4b was subjected to Baeyer–Villiger oxidation using mCPBA-TFA to afford nitrosulfonyl ester 6 in 93% yield. Lithium hydroxidemediated hydrolysis of nitrosulfonyl ester 6 afforded quaternaryg-nitro-g-sulfonyl carboxylic acid 7 in 84% yield. More importantly,the ester 6 was successfully transformed to hydroxamic acid 8 in74% yield by treating with hydroxylamine hydrochloride.
In conclusion, conjugate addition of a-nitrosulfones to vinylketones afforded a-nitro-d-ketosulfones in excellent yield andenantioselectivity in the presence of as low as 0.2 mol% quinine–squaramide organocatalyst. The feasibility of scale up of theenantioselective conjugate addition as well as practical applicationof the products in the enantioselective synthesis of carboxylic acidsand hydroxamic acids have been successfully demonstrated.
INNN thanks DAE India for financial assistance and KSBthanks CSIR India for a senior research fellowship.
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15 K. Bera and I. N. N. Namboothiri, Org. Lett., 2012, 14, 980.16 K. Bera and I. N. N. Namboothiri, Adv. Synth. Catal., 2013, 355, 1265.17 (a) J. P. Malerich, K. Hagihara and V. H. Rawal, J. Am. Chem. Soc.,
2008, 130, 14416; review: (b) J. Aleman, A. Parra, H. Jiang andK. A. Jørgensen, Chem.–Eur. J., 2011, 17, 6890.
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Fig. 3 X-ray structure of 4i and proposed mechanistic model.
Scheme 1 Enantioselective synthesis of hydroxamic acid.
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